In medical emission tomography, the physical functions of body organs and related biochemical processes such as iodine metabolism of the thyroid, glucose metabolism of the brain and heart, and blood flow to the heart, muscle and brain is measured by the emission of photons (gamma rays) from radioactive isotopes administered to the patent. Specifically, the isotopes selected have decay characteristics which produce gamma rays having defined energy characteristics, the intensity and location of which are recorded as data and imaged by a nuclear camera.
There are medical applications where two or more isotopes are desired to be imaged simultaneously. Specifically, thallium (i.e., thallium 201 as thallus chloride) has been used in imaging the heart to localize myocardial infarcts. Technetium (i.e., technetium 99m) has been used in lung imaging. Simultaneous imaging of both isotopes is desired so that technetium can be utilized to record the heart's response to stress and thallium can be utilized to image the heart's rest or redistribution state to show its recovery from stress.
Simultaneous data acquisition and imaging of two or more spectrally close isotopes has heretofore not been successfully accomplished by Anger cameras using scintillation crystals such as NaI. Improvements have, however, continued to be made to the resolution of nuclear cameras to better define the pulse height spectrum produced by the gamma rays. Energy resolutions of gamma camera systems have improved from a variance range of about 10.5% to 12% @ 140 KeV to a range today of about 8.5% to 9.75% @ 140 KeV. This is a significant improvement in terms of what is required mathematically of the photopeak window to permit imaging. In theory, and until the present invention, it is now possible to simultaneously image spectrally close isotopes such as technetium and thallium. While the camera resolution has improved to where the camera can now accurately discriminate between gamma ray emissions at differential levels as low as about 50 KeV, it has been discovered through experimentation that accurate scintigrams have not been produced because of scattered and secondary cross channel radiation occurring within the collimator which spills scattered photons and K-shell X-rays into the scintillation crystal. Although the quantity of the cross channel radiation is not significant, it has been determined, surprisingly, that such radiation adversely effects the photopeaks preventing accurate imaging of at least one of the gamma rays and/or distinguishing between the two radiations.
Until the invention, the problem has not been resolved. In conventional practice, Anger cameras are fitted with removable collimators having varying thickness for collimating X-rays and gamma rays of varying energies. Collimators are typically classified as thin, medium and thick with the thick collimators weighing in excess of 500 lbs for collimating X-rays having high radiation energies. Within the art, various devices have been proposed to vary the length of the tubular passageways so that one collimator can be used for all radiation energies. See for example Wunderlich U.S. Pat. No. 4,348,591, Heller U.S. Pat. No. 4,528,453 or Larsson U.S. Pat. No. 4,597,096. Such collimators do not address and are not capable of simultaneously analyzing two or more isotopes.
It has long been known in radiology to use filters to "harden" the radiation beam so that the X-ray beam has a higher percentage of higher energy, more penetrating photons. It has also been known with monoenergetic radiation such as cobalt 60 gamma rays, that filters are not used to harden the radiation (since Co60 evidences two discrete energies) but are used as beam-flattening filters. Various compound filters such as the Thoreaus filter are used to increase the radiation exposure rate (The Fundamentals of X-Ray and Radium Physics, 7th Edition, Joseph Selman, published by Charles C. Thomas, 1985, pages 207-209). Filters are placed in the path of the radiation beam to attenuate or flatten the beam. Placing filters in the path of the gamma radiation for photon emission tomographs (PET) would simply attenuate the radiation making detection more difficult.
A number of mathematical techniques and formula (pulse shape discrimination, i.e., PSD) have been utilized to differentiate pulses produced by different types of particles in the source detector. In fact, all cameras use some type of band pass methodology to provide pulse height discrimination. To the extent the particles exhibit different energies, such techniques are acceptable. However, if the particles exhibit similar energies, the techniques cannot, by definition, discriminate. Thus, the discriminators serve only to reduce the primary and a small fraction of the secondary emissions produced. The active window width secondary emission faction remains unchanged. Furthermore, because the cross channel radiation occurs sporadically or randomly in time, it is not possible to use statistical techniques, conventionally known as "binning", to artificially modify the pulse height shape to account for variations.
One approach described in the literature eliminates or reduces that portion of the energy spectrum detected in a Na(Tl) scintillator or a Ge(Li) detector attributed to the Compton effect which is known as a Compton-suppression spectrometer. The Compton-suppression spectrometer uses two detectors which are operated in anticoincidence. One specific arrangement uses a larger NaI(Tl) scintillator which surrounds a Ge(Li) detector. When the two detectors are operated in anticoincidence, the center Ge(Li) detector will consist of pulses that result from total energy absorption in that detector. (Measurement and Detection of Radiation, Nicholas Tsoulfanidis, published by Hemisphere Publishing Corporation, 1983, pages 357-360). While this approach may eliminate some secondary radiation it does not take into account scattering and requires two detectors and associated electronics. Further, the energy spectrum attributed to the Compton effect, while always a nuisance, does not cause the problem preventing simultaneous imaging, and is otherwise addressed by the band pass discrimination methods discussed above.